This invention relates to a flame retardant epoxy resin composition for semiconductor encapsulation which is effectively moldable and cures into a product having flame retardance and moisture-proof reliability despite the absence of bromides (e.g., brominated epoxy resins) and antimony compounds (e.g., antimony trioxide). It also relates to a semiconductor device encapsulated with a cured product of the composition.
The current mainstream of semiconductor devices resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resins have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. Semiconductor devices are now used in every area of the modern society, for example, in electric appliances and computers. As a guard against accidental fire, the semiconductor devices are required to be flame retardant.
In semiconductor encapsulating epoxy resin compositions, halogenated epoxy resins combined with antimony trioxide (Sb2O3) are often included in order to enhance flame retardance. This combination of a halogenated epoxy resin with antimony trioxide has great radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect.
However, the halogenated epoxy resins generate noxious gases during combustion, and antimony trioxide has powder toxicity. Given their negative impact on human health and the environment, it is desirable to entirely exclude these fire retardants from resin compositions.
In view of the above demand, studies have been conducted on the use of hydroxides such as Al(OH)3 and Mg(OH)2 or phosphorus based fire retardants such as red phosphorus and phosphates in place of halogenated epoxy resins and antimony trioxide. Unfortunately, various problems arise from the use of these alternative compounds. The hydroxides such as Al(OH)3 and Mg(OH)2 have less flame retardant effects and must be added in larger amounts in order that epoxy resin compositions be flame retardant. Then the viscosity of these compositions increases to a deleterious level to molding, causing molding defects such as voids and wire flow. On the other hand, the phosphorus-containing fire retardants such as red phosphorus and phosphates added to epoxy resin compositions can be hydrolyzed to generate phosphoric acid when the semiconductor devices are exposed to hot humid conditions. The phosphoric acid generated causes aluminum conductors to be corroded, detracting from reliability.
To solve these problems, Japanese Patent No. 2,843,244 proposes an epoxy resin composition using a flame retardant having red phosphorus coated with a surface layer of SixOy although it still lacks moisture-proof reliability. JP-A 10-259292 discloses an epoxy resin composition in which a cyclic phosphazene compound is used in an amount to give 0.2-3.0% by weight of phosphorus atoms based on the total weight of other constituents excluding the filler. To provide flame retardance, a substantial amount of the cyclic phosphazene compound must be added to the epoxy resin composition, which can cause ineffective cure and a lowering of electrical resistance in a high-temperature environment.
An object of the invention is to provide a flame retardant epoxy resin composition for semiconductor encapsulation which is effectively moldable and cures into a product having improved flame retardance and moisture-proof reliability despite the absence of bromides (e.g., brominated epoxy resins) and antimony compounds (e.g., antimony trioxide). Another object is to provide a semiconductor device encapsulated with the composition in the cured state.
We have found that a semiconductor encapsulating, flame retardant epoxy resin composition comprising (A) an epoxy resin, (B) a curing agent, (C) an inorganic filler, (D) a molybdenum ingredient having zinc molybdate supported on an inorganic filler, (E) a first phosphazene compound of the average compositional formula (1), and (F) a second phosphazene compound of the average compositional formula (2), shown below, as essential components, and substantially free of bromides and antimony compounds is effectively moldable and cures into a product having improved flame retardance and moisture-proof reliability. A semiconductor device encapsulated with the epoxy resin composition in the cured state is improved in flame retardance and moisture-proof reliability.
Accordingly, the present invention provides a semiconductor encapsulating, flame retardant epoxy resin composition comprising as essential components, (A) an epoxy resin, (B) a curing agent, (C) an inorganic filler, (D) a molybdenum ingredient having zinc molybdate supported on an inorganic filler, (E) a first phosphazene compound having the average compositional formula (1), and (F) a second phosphazene compound having the average compositional formula (2), the epoxy resin composition being substantially free of bromides and antimony compounds. 
Herein X is a single bond or a group selected from among CH2, C(CH3)2, SO2, S, O, and O(CO)O, subscripts d, e and n are numbers satisfying 0xe2x89xa6dxe2x89xa60.25n, 0xe2x89xa6e less than 2n, 2d+e=2n, and 3xe2x89xa6nxe2x89xa61000. 
Herein Y is OH, SH or NH2, R1 is selected from among alkyl and alkoxy groups having 1 to 4 carbon atoms, NH2, NR2R3 and SR2, each of R2 and R3 is hydrogen or an alkyl group having 1 to 4 carbon atoms, and n is a number satisfying 3xe2x89xa6nxe2x89xa61000.
Also contemplated herein is a semiconductor device encapsulated with a cured product of the epoxy resin composition.
As stated above, the epoxy resin composition of the invention is substantially free of bromides and antimony compounds. Although brominated epoxy resins and antimony trioxide are generally included in conventional epoxy resin compositions in order to impart flame retardance thereto, the epoxy resin composition of the invention clears the flame retardant specifications, UL-94, V-0 without resorting to brominated epoxy resins and antimony trioxide.
In the prior art, studies were conducted on the use of hydroxides such as Al(OH)3 and Mg(OH)2 or phosphorus-based fire retardants such as red phosphorus and phosphates in place of the brominated epoxy resins and antimony trioxide. Unfortunately, these known alternative flame retardants have the common drawback that they are less resistant to water, especially at elevated temperatures, so that they are dissolved and decomposed to increase impurity ions in the extracting water. As a consequence, if semiconductor devices encapsulated with conventional flame retarded epoxy resin compositions substantially free of bromides and antimony compounds are kept in a hot humid environment for a long period of time, the aluminum conductors in the devices can be corroded, detracting from moisture-proof reliability.
Addressing the above concerns, we have discovered that a semiconductor encapsulating epoxy resin composition using as a flame retardant a combination of three components, (D) a molybdenum ingredient having zinc molybdate supported on an inorganic filler, (E) a first phosphazene compound of the average compositional formula (1), and (F) a second phosphazene compound of the average compositional formula (2) does not increase impurity ions in the extracting water, is effectively moldable and cures into a product having improved flame retardance and moisture-proof reliability. These three components of flame retardant are fully resistant to water and do not increase impurity ions in the extracting water. When these three components are used alone, there arises the problem that the epoxy resin composition is given insufficient flame retardant effects, loses flow or becomes less curable. The use of the above three components (D), (E) and (F) in combination as a flame retardant allows the amounts of the individual components added to be minimized, and thus eliminates the above-mentioned problems during molding. Then the flame retardant epoxy resin composition of the invention cures into a product having significantly improved flame retardance and moisture-proof reliability.
Component (A) in the epoxy resin composition of the invention is an epoxy resin which is not critical. Illustrative examples of suitable epoxy resins include novolac type epoxy resins, cresol novolac type epoxy resins, triphenolalkane type epoxy resins, aralkyl type epoxy resins, biphenyl skeleton-containing aralkyl type epoxy resins, biphenyl type epoxy resins, dicyclopentadiene type epoxy resins, heterocyclic epoxy resins, naphthalene ring-containing epoxy resins, bisphenol A type epoxy compounds, bisphenol F type epoxy compounds, and stilbene type epoxy resins. These epoxy resins may be employed alone or in combination of two or more. Halogenated, especially brominated, epoxy resins are excluded.
The epoxy resin used herein preferably has a hydrolyzable chlorine content of up to 1,000 ppm, especially up to 500 ppm and sodium and potassium contents of up to 10 ppm for each. If the content of hydrolyzable chlorine is more than 1,000 ppm or if the content of sodium or potassium is more than 10 ppm, moisture resistance may degrade when the encapsulated semiconductor device is long held in a hot humid environment.
Component (B) is a curing agent which is not critical as well. Most often, phenolic resins are used as the curing agent. Illustrative examples of typical phenolic resin curing agents include phenolic novolac resins, naphthalene ring-containing phenolic resins, aralkyl type phenolic resins, triphenolalkane type phenolic resins, biphenyl skeleton-containing aralkyl type phenolic resins, biphenyl type phenolic resins, alicyclic phenolic resins, heterocyclic phenolic resins, and bisphenol-type phenolic resins such as bisphenol A and bisphenol F type phenolic resins. These phenolic resins may be employed alone or in combination of two or more.
Like the epoxy resin, the curing agent used herein preferably has sodium and potassium contents of up to 10 ppm for each. If the content of sodium or potassium is more than 10 ppm, moisture resistance may degrade when the encapsulated semiconductor device is long held in a hot humid environment.
The epoxy resin and the curing agent may be admixed in any desired proportion. Preferably they are used in such amounts that the molar ratio of phenolic hydroxyl groups in the curing agent and the second phosphazene compound of formula (2) to epoxy groups in the epoxy resin is from 0.5 to 1.5, more preferably from 0.8 to 1.2.
In the practice of the invention, a curing accelerator is preferably used to promote the curing reaction between the epoxy resin and the curing agent. The curing accelerator may be any suitable substance that promotes the curing reaction. Illustrative, non-limiting examples of curing accelerators that may be used include phosphorus compounds such as triphenylphosphine, tributylphosphine, tri(p-methylphenyl)phosphine, tri(nonylphenyl)phosphine, triphenylphosphine triphenylborane, and tetraphenylphosphine tetraphenylborate; tertiary amine compounds such as triethylamine, benzyldimethylamine, xcex1-methylbenzyldimethyl-amine, and 1,8-diazabicyclo[5.4.0]undecene-7; and imidazole compounds such as 2-methylimidazole, 2-phenylimidazole, and 2-phenyl-4-methylimidazole.
The curing accelerator is blended in an effective amount to promote the curing reaction between the epoxy resin and the curing agent (typically phenolic resin), desirably in an amount of about 0.1 to 5 parts, more desirably about 0.5 to 2 parts by weight per 100 parts by weight of components (A), (B), (E) and (F) combined, independent of whether it is a phosphorus compound, tertiary amine compound or imidazole compound as stated above.
The inorganic filler (C) included in the epoxy resin compositions of the invention may be any suitable inorganic filler commonly used in epoxy resin compositions. Illustrative examples include silicas such as fused silica and crystalline silica, alumina, silicon nitride, aluminum nitride, boron nitride, titanium oxide, and glass fibers.
No particular limit is imposed on the mean particle size and shape of these inorganic fillers as well as the amount thereof. To enhance the flame retardance, the inorganic filler is preferably contained in the epoxy resin composition in a larger amount insofar as this does not compromise moldability. With respect to the mean particle size and shape of the inorganic filler, spherical fused silica having a mean particle size of 5 to 30 xcexcm is especially preferred. The amount of the inorganic filler (C) loaded is preferably 400 to 1,200 parts, more preferably 500 to 1,000 parts by weight per 100 parts by weight of components (A), (B), (E) and (F) combined.
The inorganic filler used herein is preferably surface treated beforehand with a coupling agent such as a silane coupling agent or a titanate coupling agent in order to increase the bonding strength between the resin and the inorganic filler. The preferred coupling agents are silane coupling agents including epoxy silanes such as xcex3-glycidoxy-propyltrimethoxysilane, xcex3-glycidoxypropylmethyldiethoxy-silane, and xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino silanes such as N-xcex2-(aminoethyl)-xcex3-aminopropyl-trimethoxysilane, xcex3-aminopropyltriethoxysilane, and N-phenyl-xcex3-aminopropyltrimethoxysilane; and mercapto silanes such as xcex3-mercaptosilane. No particular limitation is imposed on the amount of coupling agent used for surface treatment or the method of surface treatment.
The epoxy resin composition of the invention contains (D) a molybdenum ingredient having zinc molybdate supported on an inorganic filler as a flame retardant.
To develop satisfactory flame retarding effects, zinc molybdate should preferably be uniformly dispersed in the epoxy resin composition. For improving the dispersion, the molybdenum ingredient having zinc molybdate previously supported on an inorganic filler such as silica or talc is optimum.
Suitable inorganic fillers on which zinc molybdate is supported include silicas such as fused silica and crystalline silica, talc, alumina, silicon nitride, aluminum nitride, boron nitride, titanium oxide, zinc oxide, and glass fibers. The inorganic filler should preferably have a mean particle size of 0.1 to 40 xcexcm, and more preferably 0.5 to 15 xcexcm and a specific surface area of 0.5 to 50 m2/g, and more preferably 0.7 to 10 m2/g. It is noted that the mean particle size can be determined as the weight average value or median diameter by the laser light diffraction technique, for example, and the specific surface area is determined by the BET adsorption method, for example.
In the molybdenum ingredient having zinc molybdate supported on the inorganic filler, the content of zinc molybdate is preferably 5 to 40% by weight, more preferably 10 to 30% by weight. Less contents of zinc molybdate may fail to provide satisfactory flame retardance whereas excessive contents may detract from flow during molding and curability.
The molybdenum ingredient in the form of zinc molybdate on inorganic filler is commercially available under the trade name of KEMGARD 1260, 1261, 911B and 911C from Sherwin-Williams Co.
An appropriate amount of the molybdenum ingredient blended is 3 to 100 parts, more preferably 5 to 50 parts by weight per 100 parts by weight of components (A), (B), (E) and (F) combined. Less than 3 parts of the molybdenum ingredient may fail to provide satisfactory flame retardance whereas more than 100 parts may detract from flow and curability.
The molybdenum ingredient comprising zinc molybdate supported on inorganic filler is added in such amounts that 0.1 to 20 parts, especially 0.2 to 20 parts by weight of zinc molybdate is present per 100 parts by weight of the epoxy resin and the curing agent combined. Less than 0.1 part by weight of zinc molybdate may fail to provide satisfactory flame retardance whereas more than 20 parts by weight of zinc molybdate may detract from flow and curability.
Included in the semiconductor encapsulating, flame retardant epoxy resin composition of the invention are (E) a first phosphazene compound and (F) a second phosphazene compound.
The first phosphazene compound (E) has the average compositional formula (1): 
wherein X is a single bond or a group selected from among CH2, C(CH3)2, SO2, S, O, and O(CO)O, subscripts d, e and n are numbers satisfying 0xe2x89xa6dxe2x89xa60.25n, 0xe2x89xa6e less than 2n, 2d+e=2n, and 3xe2x89xa6nxe2x89xa61000.
As compared with epoxy resin compositions having phosphorus base flame retardants (e.g., red phosphorus and phosphates) added, the semiconductor encapsulating, flame retardant epoxy resin composition of the invention having the first phosphazene compound of formula (1) added affords a cured product having resistance to hot water extraction and especially, high moisture-proof reliability. The use of the phosphazene compound of formula (1) in combination with the molybdenum ingredient achieves more enhanced flame retarding effects.
In formula (1), n is in the range of 3 to 1000, preferably 3 to 10. For synthesis, n=3 is most preferred.
The proportion of d and e is to satisfy 0xe2x89xa6dxe2x89xa60.25n, 0xe2x89xa6e less than 2n, and 2d+e=2n. If 0.25n less than d, then the phosphazene compound has a higher softening point due to more intermolecular crosslinks, and thus becomes less miscible with the epoxy resin, failing to achieve the desired flame retarding effects. The proportion of d is preferably 0.015nxe2x89xa6dxe2x89xa60.25n. The proportion of e is 0xe2x89xa6e less than 2n, and 1.5nxe2x89xa6exe2x89xa61.97n is desired to achieve a higher level of flame retardance without compromising other desired properties.
It is understood that when X is a single bond, 
is represented as 
The second phosphazene compound (F) has the average compositional formula (2): 
wherein Y is OH, SH or NH2, R1 is selected from among alkyl and alkoxy groups having 1 to 4 carbon atoms, NH2, NR2R3 and SR2, each of R2 and R3 is hydrogen or an alkyl group having 1 to 4 carbon atoms, and n is a number satisfying 3xe2x89xa6nxe2x89xa61000.
In formula (2), R1 is selected from among alkyl and alkoxy groups having 1 to 4 carbon atoms, NH2, NR2R3 and SR2. If R1 is an alkyl or alkoxy group having more than 4 carbon atoms, the flame retarding effects achieved with this compound decline as the number of carbon atoms increases. The same applies to R2 and R3. For this reason or other, R1 is preferably methyl, methoxy, amino or dimethylamino.
The first phosphazene compound of the average compositional formula (1) exerts good flame retarding effects. However, due to the lack of reactive groups, as the addition amount thereof is increased, it can detract from curability and electrical properties at elevated temperatures. On the other hand, the second phosphazene compound of the average compositional formula (2) has positive effects on cure and electrical properties at elevated temperatures due to the inclusion of substituent groups capable of reaction with the epoxy resin. However, once it has reacted with the epoxy resin, it is taken into the matrix and kept substantially immobile, sometimes failing to achieve the desired flame retarding effects upon burning. It has been found that in order to meet all the requirements of flame retardance, cure and electrical properties, the first and second phosphazene compounds (E) and (F) are preferably mixed in a ratio E/F between 50/50 and 90/10, especially between 50/50 and 80/20, as expressed by a weight ratio of phosphorus atoms contained in the respective compounds.
The total amount of the first and second phosphazene compounds added, that is, the amount of components (E) and (F) combined is preferably 1 to 50% by weight, more preferably 2 to 20% by weight based on the total weight (100% by weight) of components (A), (B), (E) and (F) combined. Less than 1% by weight of the phosphazene compounds may fail to provide the desired flame retarding effects whereas more than 50% by weight of the phosphazene compounds may detract from fluidity. The content of phosphorus in the phosphazene compounds, i.e., components (F) and (F) is preferably 0.05 to 7% by weight, more preferably 0.1 to 2% by weight, based on the entire epoxy resin composition.
The semiconductor encapsulating, flame retardant epoxy resin composition of the invention may further include other flame retardants, for example, hydroxides such as aluminum hydroxide and magnesium hydroxide, inorganic compounds such as zinc borate and zinc stannate, and silicone compounds as long as the objects and benefits of the invention are not impaired. Note that antimony compounds such as antimony trioxide are excluded.
The inventive epoxy resin composition may also include various additives, if necessary and as long as the objects of the invention are not impaired. Illustrative examples include stress-lowering additives such as thermoplastic resins, thermoplastic elastomers, synthetic organic rubbers, and silicones; waxes such as carnauba wax, higher fatty acids and synthetic waxes; colorants such as carbon black; and halogen trapping agents.
The inventive epoxy resin compositions may be prepared as a molding material by compounding the epoxy resin, curing agent, inorganic filler, molybdenum ingredient, phosphazene compounds, and optional additives in predetermined proportions, thoroughly mixing these components together in a mixer or other appropriate apparatus, then melting and working the resulting mixture using a hot roll mill, kneader, extruder or the like. The worked mixture is then cooled and solidified, and subsequently ground to a suitable size so as to give a molding material.
The resulting epoxy resin compositions of the invention can be effectively used for encapsulating various types of semiconductor devices. The method of encapsulation most commonly used is low-pressure transfer molding. The epoxy resin composition of the invention is preferably molded at a temperature of about 150 to 180xc2x0 C. for a period of about 30 to 180 seconds, followed by post-curing at about 150 to 180xc2x0 C. for about 2 to 16 hours.
The semiconductor encapsulating, flame retardant epoxy resin compositions of the invention are effectively molded and cure into products which have excellent flame retardance and reliability against moisture. The compositions have no ill effects on human health and the environment because they do not contain bromides such as brominated epoxy resins and antimony compounds such as antimony trioxide. The semiconductor devices encapsulated with the cured epoxy resin compositions of the invention remain fully flame retardant and reliable against moisture and are thus of great worth in the industry.